1. Field of the Invention
The invention relates to a pilot valve, especially for servo valves.
2. Description of Related Art
In electro-hydraulic servo valves the hydraulic output (fluid flow or pressure) is proportional to the electrical input signal. In the case of larger pressure fluid volume flows, usually, two-stage servo valves are employed in which a main stage is adjusted via a hydraulic pilot stage. Such a two-stage servo valve is distributed by the applicant, for instance, under the product name 4WS.2DM. The pilot stage substantially consists of a control motor by means of which the position of a bounce plate is variable between two control nozzles. By approximation of the bounce plate to one of the control nozzles a control oil flow is reduced by the latter, while the control oil flow is increased by the other control nozzle. In the flow path of the control oil to each nozzle a respective diaphragm is arranged so that also the pressure drop is appropriately reduced or increased via the respective diaphragms by varying the control oil volume flow.
The pressure adjusting downstream of the diaphragms is tapped off by means of a control duct and guided to control surfaces pressurizing a main slide of a main stage on the front side so that the main slide is displaced into a control position in response to the pressure difference occurring. The bounce plate and the control nozzles thus act as hydraulic booster by means of which the displacement of position caused by the control of the actuating motor is converted to a pressure difference.
In some applications it is necessary, for instance when taking the system into operation in accident (emergency stop) or in troubleshooting, to put the servo pilot valve manually into a predetermined position so that the main slide is appropriately adjusted and the system is switched to be pressureless or the like.
Servo adjusting systems are offered by Moog under the designation D062-900 in which the mechanical emergency actuation is effected by a lever disposed at a cap of the servo valve enclosing the actuating motor and acting directly upon the actuating motor.
It is a drawback of said solution that in the case of violent operation of the lever the actuating motor can be damaged and that the lever mimic is also relatively sensitive to pollution and vibrations of the system.
Compared to this, the object underlying the invention is to provide a pilot valve, especially for a servo valve, which has an elegant design while permitting a reliable manual operation.